This invention relates to a device for purifying water and in particular to a water crystallization type purifier in which pure ice crystals are formed in a contaminated water. The ice crystals are separated from the contaminated water and melted to provide a supply of pure water.
There are numerous methods of purifying water. Three primary methods are practical for processing large quantities of contaminated source water: distillation, electrodialysis and crystallization.
Distillation has had wide practical application in purifying contaminated or salt waters. However, low temperature evaporation and condensation of large quantities of water require extensive capital equipment and substantial land areas for efficient operation. High temperature vapor systems require abundant fuel supplies and pressurized heat transfer systems that are subject to corrosion, scaling and failure.
In electrodialysis, two spaced permeable membranes selectively pass positive and negative ions from a salt solution therebetween, to electrodes on the other sides of the membrane. Generally size and cost limitations restrict the use of electrodialysis to limited volumes of mildly saline solutions.
Crystallization has been used in the past, but has had limited application. Because of its advantages as a low temperature process, problems of corrosion and scaling are minimal. Crystallization purification has been used to desalinize sea waters. Both continuous and batch processes have been used to produce quantities of water on large and small scales. The relatively low energy requirement compared to distillation and membrane dialysis makes crystallization an attractive general method for producing purified water. While the principle of purification through freezing has been well known, surprisingly few practical applications of the principle have been implemented.
The water purifier of this invention utilizes a cyclical freeze process to produce purified drinking water from a contaminated or non potable water source. The water purifier employs a design concept that is suitable for low volume purifiers for household use, or high volume purifiers for industrial use. The concept is useable for economical purification of water containing up to 4500 p.p.m. of contaminants. Water having much higher concentrations of contaminants can be purified by multiple passes through the purification system. Sea water, for example, having 22,900 p.p.m. of impurities, can be purified by three passes to obtain potable water of high quality.
The devised water purifier uses a hybred spray process to produce fine droplets that form ice crystals on a refrigerated contact surface. As the ice crystals accumulate they are continually washed by the unfrozen portion of the continuous spray. This process flushes away a contaminated brine film that adheres to the surface of tiny platelets of crystallizing ice. The ice cake that accumulates is periodically removed and melted to provide a supply of pure water.
The hybred spray process was devised to incorporate certain phenomenae reported from a prior investigation of water refrigerant mix systems. In such systems the water is directly mixed with a refrigerant to initiate crystallization from the vaporization of the refrigerant from the water refrigerant mix. In the investigation of a technique for mixing by colliding separate sprays of refrigerant and water, a fine spray of freezing droplets is produced. The crystals formed were substantially smaller than those produced by liquid mixtures of a feedwater and a liquid refrigerant in a crystallizer. The smaller the crystal the less likely that surface films become trapped between the platelets of accumulating crystals.
While direct contact methods of water purification are suitable for large volumes of water, the difficult process of separating the ice crystals from the brine in the resulting slurry makes the process impractical for smaller units. Additionally there is a reluctance to directly mix water with a refrigerant when potable water is sought. This disadvantage is obviated by the use of a heat transfer surface separating the water from the refrigerant.
The problem with surface contact systems, however, is the added cost of the heat transfer components and the additional energy consumption required. In such units the surface is directly washed by a continuous stream of contaminated water which transfers to the exchange surface large amounts of latent heat acquired during a cycling process.
Because surface area is a primary limitation to efficient heat transfer for large volumes of water, applicant has sought to maximize the effective area by two means. First, the contact surface area is increased by the use of a plurality of projecting, thimble-like freeze elements. Second, the surface area of the water is increased by projecting the water at the elements in a fine spray of small droplets to enable rapid formation of seed crystals on contact with the freeze elements or the ice cake formed thereon. The cold atmosphere of the crystallizer aids in reducing the temperature of the droplets close to the freeze temperature before impact. The impact of the droplets has a wash effect, to purge contaminants on the crystals as they accumulate on the freeze elements. Additionally the orientation of the freeze elements allows the contaminated brine to effectively drip from the end of the elements.
As the efficiency begins to diminish, because of ice buildup, the freeze elements are warmed to release the capsules of caked ice. The ice is then melted providing a supply of potable water.